Electrical noise generator

A device that produces electrical noise for use in electrical
measurements. Electrical noise generators are commonly employed to measure
the noise figure or noise temperature of radio receivers. They are also
used in various other tests in radar and communications systems. Celestial
noise sources are used to calibrate large antennas.

Some standard types of noise generators are hot-wire, diode, gas-discharge
tube, hot and cold loads (terminations), and radio star. A hot-wire noise
source consists of the filament of a lamp heated by direct current. Thermal
noise having spectral density 4 kTR, where k is Boltzmann’s constant and T and R are the temperature and resistance of the filament respectively,
is generated across the terminals of the filament. A diode noise generator
utilizes the temperature-limited shot effect to generate noise. At frequencies
less than the reciprocal transit time of the diode, the noise spectral
density is 2eI, where e is the charge on the electron and I is the average
anode current. A gas-discharge noise generator, commonly referred to
as a noise tube, consists of a fluorescent light tube enclosed in a waveguide.
Noise generation is essentially thermal. The noise tube is commonly employed
at microwave frequencies. Hot and cold loads consist of well- matched
terminations, either transmission line or waveguide, held at a given
temperature by using an oven or by applying cryogenic refrigeration.
Noise generation is thermal. Common temperatures for noise-generating
terminations are nominally 80 and 300 K (minus 316 and plus 80°F).

Celestial radio sources (radio stars) are commonly employed as reference
noise sources for evaluating the characteristics of very low-noise, high-gain
space communications receiving antennas. There are a number of accurately
calibrated sources available -- the choice depending on system parameters
(frequency, antenna gain, system noise temperature, elevation angle, and so forth) and physical location. The most common radio sources employed
are Cassiopeia A, Taurus A, Cygnus A, and Orion A. The first three are
classified as non-thermal sources in which radiation results from relativistic
electrons interacting with an interstellar magnetic field. The electrons
are rotated in a plane perpendicular to the magnetic field direction, and radiation is characterized by a component polarized parallel to that
plane. The polarized component is small, however, and the major portion
of the radiation is unpolarized. The non-thermal sources have flux densities
which decrease with increasing frequency and , consequently, tend to have
a cutoff frequency above which they are not usable. Orion A is a thermal
source in which radiation occurs from a hot, ionized cloud. Orion A has
a constant flux density at frequencies above 2 GHz. As a point of reference,
a typical value of thermal noise received from Cassiopeia A by a 10-m-diameter
(33-ft) antenna operating in the C band (4-6 GHz) would be about 150
K (minus 190 F).